Co/Al2O3 catalysts have been prepared with conventional impregnation and sol-gel methods to vary the chemical reactivity of the alumina support. The material system has been investigated with X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), diffuse reflectance Fourier transform spectroscopy (DRIFT), Brunauer-Emmett-Teller (BET) method, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR) (with thermogravimetric analysis (TGA) and differential thermal analysis (DTA)), and gas chromotography (GC) for the catalytic oxidation of CO. It had been found that the reactivity of the support changes the surface structure and chemical composition of catalysts significantly. When the metal-support interaction is weak, Co3O4 is a predominant surface phase (which is interfaced by a "cobalt surface phase"). With an increase in support reactivity, CoO and CoAl2O4 are found to be present on the surface. The cobalt content on the surface decreases upon the increase in support reactivity and surface area. At the high atomic ratio of Al/Co, there are more =Al-O-H bonds that show higher basicity. Reducibility of the phases observed decreases on the order of Co3O4, COO, and CoAl2O4. It is found that the removal of CO on metallic sites is much easier in the catalyst with lower metal-support interactions. At room temperature, reactions between the adsorbed CO and lattice oxygen have been observed in all samples. The catalytic activity for the CO combustion reaction increases with the decrease in metal-support interactions of the catalysts. Possible causes for the above observations have been addressed.